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THIN PLATES OF METAL. 



BY 



PKOF. T.°EGLESTON, Ph.D., 

SCHOOL OF MINES, COLUMBIA COLLEGE, NEW YORK CITY. 



From Vol. VII. Transactions of the American Institute op 

Mining Engineers. 
Issued at the Secretary's Office, Lafayette College, Easton, Pa 



PRINTED FOR THE AUTHOR. 

1 8 7 9. 



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THIN PL A TES OF METAL. 

BY PROF. T. EGLESTON, PH.D., SCHOOL OF MINES, COLUMBIA COLLEGE, 

NEW YORK CITY. 

The importance of having perfectly pure metals has led me to 
present to the Institute a record of some of the trials that have been 
made to obtain these metals, and also to show one of the largest 
specimens of extremely thin metal plate which has ever been made. 

The malleability of metals varies generally directly with their 
purity, and hence it is only with very pure metals that thin sheets 
can be obtained. . The competition among different manufacturers 
has been so great at times as to lead to expensive and apparently 
useless experiments in obtaining in the first instance very thin sheets, 
and afterwards very large and thin sheets of metal, apparently with 
no other purpose than that of being able to say that they had pro- 
duced the thinnest or the largest thin sheet that had ever been made. 
These experiments were at first confined exclusively to iron, their 
object being to show the great dexterity of manipulation, as well as 
the purity of the metals manufactured. They have since been ex- 
tended to almost all metals by electrical action, and have gone far 
beyond the limits of what was possible with purely mechanical means. 

The processes which have been used are rolling, hammering, elec- 
trical deposition in a vacuum, and lastly, electrical deposition on 
plates, easily soluble in acids. The first and last of these methods 
have been known and practiced for a very long time, having been 
used to make thin sheets of almost all the metals. The other is of 
quite recent application. 

In the iron manufacture, the strife to produce these thin sheets was 
commenced in the year 1865, by the Sligo Iron Works writing to a 
firm in Birmingham, England, on a sheet of iron containing 270 
square centimeters, and weighing only 4.469 grams. For some time 
this was considered to be the thinnest sheet iron that could be made. 
T. W. Booker & Co., of Cardiff, England, however, produced a few 
months afterward, a sheet of the same size, weighing only 4.015 
grams. This was succeeded by one rolled by Neville and Everitt, 
of Llannelly, weighing only 3. 174 grams. " This was followed by one 
from Hallam, of Swansea, which was 283 square centimeters in size, 
and weighed but 2.979 grams ; and this by one rolled by R. Wil- 

1 



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2 THIN PLATES OF METAL. 

Hams & Co., of West Bromich, was 445 square centimeters, and 
weighed 3.173 grams. This was succeeded by the Hope Society of 
Tipton, which produced one measuring 1425 square centimeters, and 
weighing 11.529 grams. Finally the Upper Forest Tin Works near 
Swansea, produced a sheet measuring 155 square centimeters, and 
weighing 1.296 grams, which required 1888 together to make one 
decimeter in thickness. The thinnest tissue paper is about four times 
as thick as this sheet of iron. 

Recently Prof. Wright, of New Haven, has produced by means of 
electrical discharge, perfectly transparent films of iron and of mag- 
netic oxide, but only over a very small surface. 

No application has ever been found for such thin metal as this, 
but the skill necessary to produce it has led to great improvements 
in the machinery by which sheets of metal are rolled, and a decided 
advance in the quality of the iron. 

The next step was to produce thin sheets of metals of all kinds, 
which led Prof. Wright* to invent a method by which he could pro- 
duce films of almost any degree of transparency of any metal, depos- 
iting them with sufficient certainty and accuracy to make either trans- 
parent or opaque films. The opaque ones he has applied to the 
manufacture of speculum mirrors, depositing on glass films of gold, 
0.000183 mm. thick, and of platinum, 0.000174 mm. 

The special object, however, which I have in making this commu- 
nication, is to show to the members of the Institute a film of trans- 
parent gold, which is one of the largest, and at the same time one of 
the' thinnest ever produced. It is 5.7 millimeters square, and by 
transmitted light is perfectly transparent, and of a pale yellow color, 
except in spots where the film has been purposely doubled, where it 
is a very light green. This film was prepared by Mr. A. E. Outer- 
bridge, of the United States Mint, in Philadelphia. It is estimated 
that its thickness is not more than 0.0001 mm. to 0.00015 mm, and 
it is 15,000 times thinner than ordinary printing-paper ; the diffi- 
culty of asserting the thickness is owing to the fact that its weight 
does not appreciably affect the balance. It is not more than T -J part 
of a single undulation of a green ray of light. It is so extremely 
transparent that it does not reflect the full golden color, partly be- 
cause the gold still retains a trace of copper, which gives it a reddish 
tinge. 

The method of preparing the plates was by depositing the gold 

* Sillinian's Journal, 3d Series, vol. 13 and 14. 

; 

JO.?., 



THIN PLATES OF METAL. 6 

from a galvanic battery on a sheet of thin copper, rolled down to a 
thickness of 0.005 mm. This was then cleaned and carefully bur- 
nished, and placed in the bath ; when the gold had been deposited it 
was removed, and immersed in weak nitric acid for several days, 
dissolving out in this way almost the whole of the copper, while the 
gold floated on the surface ; it was then floated upon glass, and 
placed within two plates. In order to ascertain the weights of 
previous films which were thicker, the copper was weighed before 
immersion in the battery, then taken out and reweighed, the differ- 
ence giving the weight in gold, the calculation being based on the 
weight of a cubic decimeter of pure gold. 

It is found that in this way gold may be spread over a space many 
times larger than the thinnest gold that can be prepared by beating. 
It is remarkable on this specimen, that in rolling the copper, imper- 
fections in the rolls produced very slight irregularities in the copper 
film. These are all reproduced in the shape of fine strise on the gold, 
which was floated between the glass ; probably owing to the fact that 
the copper was more compressed in the direction of these lines, and 
had thus been a better conductor, so that the gold was deposited there 
more rapidly than over the rest of the surface. When the plate is of 
appreciable thickness, these irregularities can be burnished out. 

With a plate so thin as this, nothing can be done. It cannot be 
handled or touched, except floating on water,' and then only with a 
fine camel's hair pencil. It is so perfectly transparent that the finest 
print can be seen through it without the least difficulty. 






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